1. Field of the Invention
The present invention relates to processes for synthesis of isobornyl (meth)acrylate by reacting camphene with (meth)acrylic acid in the presence of sulfuric acid and at least one compound having an inhibiting action.
2. Discussion of the Background
Monomeric isobornyl (meth)acrylate is widely used in the manufacture of varnish binding agents. An example of how this compound is synthesized is the reaction of camphene with (meth)acrylic acid in the presence of an acidic cation-exchange resin. This type of synthesis is described, for example, in European Patent Application EP A 0718271 (Atochem) and in German Unexamined Application DE-OS 4419686 (Hoechst AG); both incorporated herein by reference. Although ion-exchange catalysis appears elegant at first sight, it has the disadvantage in industrial practice that the catalyst loses activity after repeated use. One possible explanation of this loss is that a polymer film is formed on the resin despite good process control. Even regenerative cleaning and activation, for example by washing with acid followed by drying with a suitable solvent such as acetone, does not restore the original activity level, and so the catalyst must be replaced after a few cycles of use.
It is also known that camphene can be reacted with (meth)acrylic acid in the presence of concentrated sulfuric acid to obtain isobornyl (meth)acrylate. The use of concentrated sulfuric acid as catalyst is described in, for example, Japanese Unexamined Application JP-OS-54/126293; incorporated herein by reference. It should be noted immediately, however, that the yield of about 80% described in the Japanese document was unattainable by far (see Comparison Example 2).
Moreover, a very large quantity of N,N'-diphenyl-p-phenylenediamine (about 8% relative to the acrylic acid) must be used in order to prevent undesired polymerization of the (meth)acrylate. The great majority of this inhibitor must be removed from the mixture, however, because the inhibitor prevents the desired polymerization of isobornyl (meth)acrylate.
Furthermore, the proposed inhibitor has a yellow color, and so for this reason already it must be separated as completely as possible. The required cleaning steps, such as multiple distillation, lower the yield to very small quantities of pure product. Attempts can be made to replace this inhibitor by other inhibitors or mixtures of compounds having inhibiting action, but a conversion of only about 10% is obtained.